1. Field of the Invention
The present invention relates to a laser-sensitive electrophotographic lithograph printing plate material. More particularly, the present invention relates to a laser-sensitive electrophotographic lithograph printing plate material having an enhanced spectral sensitivity to semiconductor laser rays.
2. Description of the Related Art
Due to recent progress and development of small-size offset printing machines and automatic printing mechanisms, the emphasis in light printing process is now shifting to the offset printing process.
There is much research and development going on regarding offset printing plate materials, and, as a result, various improved printing plate materials are now being commercially used.
Among the conventional offset printing plate materials, an electrophotographic lithograph printing plate material having a photosensitive electrophotographic layer wherein electroconductive zinc oxide particles are dispersed as a main photoconductive material is most widely employed in the light printing industry, because it is cheap and because the process for making the printing plate from the plate material is simple and easy.
In a conventional process for producing an electrophotographic lithograph printing plate, the printing plate material is subjected to a corona charging step, an imagewise light exposing step, a developing step, and a fixing step by using a printing plate-making machine, to form visible images in the desired pattern on the photoconductive layer.
The developing step can be effected by either a dry developing method in which a mixture of a toner and a carrier consisting of an iron powder is used as a dry developing agent, or a wet developing method in which a developing liquid containing a toner dispersed in an organic solvent, for example, a petroleum solvent with a high boiling point, is used.
The wet developing method for the preparation of the printing plate is advantageous in that the reproducibility of half-tone images is high, the resolving power is excellent, the plate-making time is short, and no correction is necessary for the formed images. Therefore, the wet developing method is widely utilized for the production of electrophotographic lithograph printing plates.
In such electrophotographic lithograph printing plates, the photoconductive zinc oxide particles used as a photoconductive pigment are photosensitive only at a wavelength of about 370 nm in the ultraviolet band. Therefore in the conventional electrophotographic lithograph printing plate and offset master in which the photoconductive zinc oxide is utilized as a photoconductive pigment, it is necessary to broaden the photosensitive wavelength band of the photoconductive layer so that it conforms to the wavelength band of an exposure light source. For this purpose, many attempts have been made to add a sensitizing dye, for example, rose benzol, uranin, bromophenolphenol (BPB) or Nigrosine, to the photoconductive layer. These have been widely utilized commercially.
Generally, in a conventional printing plate-making machine, a manuscript is exposed to light irradiated from an exposure light source, for example, a halogen lamp, the reflected light from the manuscript is irradiated to a corona-charged photoconductive layer of a printing plate material, and then the light-exposed photoconductive layer is subjected to developing and drying procedures, to provide a printing plate.
Due to the recent development of various recording machines and the spread of data digitalization, a computer-to-plate type printing plate-making method is now widely utilized for the electrophotographic material. In this method, the data in the computer is edited on a CRT and the edited data is directly applied to an electrophotographic material without preparing a hard copy, to provide a printing plate.
This development of the new printing plate-making technology is promoted by the progress of laser ray technology. Particularly, a semiconductor laser is advantageous in that the laser rays can be generated in a small device at a low cost and can be directly modulated.
Recently, due to the above-mentioned advantages, a new printing plate-making system utilizing semiconductor laser rays has been developed.
The electrophotographic printing plate material usable for the laser rays must have high sensitivity at a long wavelength band of 700 to 1000 nm. Electrophotographic materials sensitive at a wavelength band of semiconductor laser rays are disclosed, for example, in U.S. Pat. Nos. 4,362,800 and 4,418,135.
The above-mentioned electrophotographic materials containing photoconductive zinc oxide are provided with a photoconductive layer spectrosensitized with a sensitizing dye, for example, a polymethine type cyanine dye, and have enhanced sensitivity at the long wavelength band.
Nevertheless, in conventional electrophotographic materials in which only the sensitizing dye is added to the photoconductive layer, the sensitivity thereof is not satisfactory in commercial use.
To enhance the sensitivity of the photoconductive layer, an attempt has been made to use a sensitizing assistant in addition to the sensitizing dye.
For example, U.S. Pat. No. 4,879,195 discloses employment of a polymethine type cyanine dye having an alkylsulfonic acid group as an N-substituent of each of the cyclic dimethylindole groups located in the two terminals of the dye molecule, together with a sensitizing assistant consisting of maleic anhydride.
The laser-sensitive lithograph printing plate material containing the sensitizing dye and the sensitizing assistant is disadvantageous in that the heat resistance of the printing plate material is poor.
The heat resistance of the printing plate material refers to a resistance of the printing plate material against a reduction in sensitivity thereof when the temperature of the printing plate material is raised.
This high heat resistance is an important property of a printing plate material, which must exhibit high durability over a long time and high stability in storage and transportation thereof.